Over recent years, there are increasing occasions in which electrophotographic copiers and printers are used in the common printing field and also in the color printing field. In the common printing field and the color printing field, there is a strong tendency to demand high quality digital black and white images or color images. For such demands, it has been proposed that highly detailed digital images are formed using a relatively short wavelength laser beam as an exposure light source. However, even when a detailed electrostatic latent image is formed on an electrophotographic photoreceptor using the relatively short wavelength laser beam of a narrowed exposure dot diameter, the current situation is that a finally formed electrophotographic image exhibits just insufficient image quality.
The reason is thought to be that photosensitive characteristics of an electrophotographic photoreceptor or charging characteristics of toner in a developer inadequately respond to characteristics required for formation of a detailed dot latent image or formation of a toner image.
Namely, with regard to the electrophotographic photoreceptor, an organic photoreceptor (hereinafter also referred to simply as a photoreceptor) conventionally developed for a relatively long wavelength laser exhibits poor sensitivity characteristics, whereby when image exposure is carried out using a relatively short wavelength laser beam of a narrowed exposure dot diameter, a formed dot latent image becomes unclear, resulting in a tendency to deteriorate dot image reproducibility.
Conventionally, as charge generating materials for a relatively short wavelength laser photoreceptor, anthanthrone-based pigments and pyranthrone-based compounds are well known (refer to Patent Document 1). However, with regard to such anthanthrone-based pigments and pyranthrone-based compounds as described in this patent publication, there is no description on special treatment therefor. Therefore, it is assumed that commercially available pigments are just simply used. Characteristics such as sensitivity, achieved when using these commercially available pigments, have made it impossible to realize adequate sensitivity or enhanced speed in high speed printers or copiers, employing relatively short wavelength lasers, which are expected to be developed from now on.
Further, to impart higher sensitivity to polycyclic quinone-based pigments, it is known that sublimation purification is carried out (refer to Patent Document 2). However, the sublimation purification method described in this patent publication is a simple sublimation purification method carried out only one time. Also, in cases in which pigments obtained via this sublimation purification are used, adequate sensitivity or enhanced speed has not yet been realized in high speed printers or copiers employing relatively short wavelength lasers.
[Patent Document 1] Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 2000-47408
[Patent Document 2] JP-A No. 57-67934